An inquiry probing the UK government's handling of the coronavirus pandemic kicked off Tuesday with the investigation mired in controversy even before the first witness is called.

The inquiry chair, retired senior judge Heather Hallett, has called for ex-prime minister Boris Johnson's unredacted WhatsApp messages and notebooks to be handed over, prompting a legal challenge from the government of his successor Rishi Sunak.

Sunak, who was finance minister during the pandemic, has denied trying to block the material, while Johnson is said to be in favor of it being shared.

Relatives of Covid-19 victims have also taken aim at the investigation saying it will be a "farce" if bereaved families are not able to testify.

Members of the Covid-19 Bereaved Families for Justice campaign group lined up outside the inquiry in central London holding pictures of their loved ones.

Launching proceedings, Hallett pledged that those who suffered during the pandemic would "always be at the heart of the inquiry".

She paid tribute to the relatives' "dignified vigil", adding that she hoped they would "understand when they see the results of the work we are doing that I am listening to them".

"Their loss will be recognised," added Hallett, who previously oversaw the coroner's inquests into the 52 people killed in the July 7, 2005 London bombings.

The UK suffered one of the worst Covid-19 death tolls in Europe with more than 128,500 fatalities recorded by mid-July 2021.

The current toll of deaths with Covid-19 on the death certificate stands at just over 227,000, according to the latest government figures.

The first phase of the inquiry is due to focus on the UK's resilience and preparedness in the face of the global health emergency.

Established by Johnson in 2021, it has been split up into six sections.

The first witnesses to give evidence in person to the inquiry will be leading epidemiologists Jimmy Whitworth and Charlotte Hammer on Wednesday.

- 'Just a farce' -

The relatives' campaign group has condemned the decision not to include any of them in the inquiry's first six-week phase.

"Without learning from the experiences of our members, how can the inquiry properly evaluate the decisions made by those in charge?" group member Barbara Herbert, who lost her husband Paul to Covid, said earlier this week.

"We are people that will be able to put reality to the theory that Hallett is testing, that has got to happen, otherwise it's just a farce," added Saleyha Ahsan, a doctor whose father Ahsan-ul-Haq Chaudry also died.

A spokeswoman for the inquiry has said Hallett had not ruled out calling testimony from bereaved people in later phases of the probe.

Public inquiries in the UK are government-funded but have an independent chair. They investigate matters of public concern, establishing facts about what happened, why and what lessons can be learned.

They do not rule on civil or criminal liability, and any recommendations are not legally binding.

Hallett has refused to back down over her request for Johnson's unredacted communications, likely to include exchanges at the heart of government relating to the ordering of lockdowns in 2020 when Sunak was in charge of the country's purse strings.

The government's Cabinet Office, the department that supports the prime minister and oversees the running of government, has questioned the relevance of the material.

But Hallett has insisted it is for her to rule what is relevant to the investigation.

A High Court judge is due to rule on the request at the end of June, with the material expected to be central to the inquiry's second phase later in 2023 on government decision-making.

Later phases will focus on how the UK health service coped with the pandemic, vaccines and therapeutics, government procurement and the impact on the care sector.

Interim reports will be published before the scheduled end of the hearings by mid-2026.